Since many people ask about this, here is how we calculated the grades:
As you probably noticed, each question got a grade between 0 and 20.
Next, as was writen on the test, the four better questions are worth 90 points (22.5 each), and the worst one only 10 points.
In order to obtain this grade, simply multiply your four better questions by 1.125, and the worse question by 0.5.
Finally, in order to get your grade, recall that the assignments are 10% of the grade and the test 90%.